Banded-iron formation (BIF), chemically precipitated sediment, typically thin bedded or laminated, consisting of 15 percent or more iron of sedimentary origin and layers of chert, chalcedony, jasper, or quartz. Such formations occur on all the continents and usually are older than 1.7 billion years. They also are highly metamorphosed. Most BIFs contain iron oxides—hematite with secondary magnetite, goethite, and limonite—and are commonly used as low-grade iron ore (e.g., as in the Lake Superior region of North America). Because BIFs apparently have not formed since Precambrian time, special conditions are thought to have existed at the time of their formation. Considerable controversy exists over BIF origin, and a number of theories have been proposed. Their formation has been variously ascribed to volcanic activity; rhythmic deposition from iron and silica solutions due to seasonal variations; oxidation of iron-rich sediments contemporaneous with deposition; and precipitation from solution as a result of special oxidation-reduction conditions.

A 2.1 billion year-old rock with banded iron formation at the National Museum of Mineralogy and …

in Precambrian time

...mineralization is found in the granulite-gneiss belts. The few exceptions include a nickel–copper sulfide deposit at Selebi-Pikwe in the Limpopo belt in Botswana that is economic to mine, and banded-iron formations in gneisses in the eastern Hubei and Liaoning provinces of northwestern China that form the foundation of a major steel industry. There are subeconomic quantities of chromitite...

...several other rock types that developed primarily during the Precambrian but rarely later. This restriction is a result of the unique conditions that prevailed during Precambrian time. For example, banded-iron formations are ferruginous sediments that were deposited on the margins of early, iron-rich oceans. Anorthosite, which consists largely of plagioclase, forms large bodies in several...